1. Field of the Invention
The present invention relates to a decision feedback equalizer for equalizing waveform distortion caused by multipath fading in a digital radio communications system. More specifically, the present invention relates to a decision feedback equalizer which includes a forward part whose signal reference point is shifted in accordance with variations of channel response. The present invention is able to attain effective removal of waveform distortion due to severe multi-path fading of multi-level quadrature amplitude modulation (QAM) signal.
2. Description of the Prior Art
A digital radio transmission is susceptible to multipath fading or the like and invites waveform distortion of the transmitted signal resulting in degradation of signal quality. In order to minimize this problem, it is the current practice to employ an automatic adaptive equalizer using a transversal filter. An adaptive equalizer may be classified into linear and non-linear types.
Before discussing the embodiments of the present invention in detail, known techniques for adaptive equalization for minimizing waveform distortion caused by multipath fading will be discussed with reference to FIGS. 1 to 4.
FIG. 1 shows a known linear equalizer in block diagram form. As shown, this arrangement includes a (2N+1)-tapped transversal filter 10 (N is a positive integer), a decision circuit 12 and a subtractor 14. A tap control signal generator 16 forms part of the transversal filter 10. As shown, the filter 10 includes a plurality of tapped delay circuits or shift registers 18 which are coupled in series and each of which has a symbol period T. The transversal filter 10 further comprises a plurality of multipliers 20 and an adder 22. Tap signals u.sub.N, . . . , u.sub.O, . . . , and u.sub.-N dispersed on a plurality of taps are respectively multiplied by corresponding tap coefficients c.sub.-N, . . . , c.sub.O, . . . and c.sub.N. The outputs of the multipliers 20 are added at the adder 22. An error siganl .epsilon. is obtained at the subtractor 14 by subtracting the output from the decision circuit 12 from the input thereto. The tap control signal generator 16 is supplied with the error signal .epsilon. and issues a plurality of tap control signals c.sub.-N, . . . , c.sub.O, . . . and c.sub.N by minimizing the mean square of the error signal .epsilon.. Since the linear equalizer shown in FIG. 1 is well known in the art, further description thereof will be omitted for brevity.
Throughout the instant specification, merely for the convenience of description, a tap coefficient and the tap corresponding thereto are denoted by the same reference numeral.
The above-mentioned linear equalization has found extensive use in terrestrial digital microwave communications systems. However, it is unable to effectively minimize deep or severe multipath distortion. Therefore, residual intersymbol interference undesirably increases. In particular, as a signal transmission rate becomes higher and signal propagation distance increases, the linear equalization is no longer sufficient to handle severe frequency selective fading wherein multipath delay spreads over a transmission symbol period.
FIG. 2 is a block diagram showing a non-linear type equalizer which takes the form of a decision feedback equalizer (DFE) denoted by reference numeral 30. The known decision feedback equalizer 30, illustrated in FIG. 2, will be discussed in detail with reference to FIG. 5 which shows a first embodiment of the present invention. Accordingly, a detailed description of the DFE 30 will not be made at this time.
The DFE shown in FIG. 2 includes a forward equalizer (FE) 32 and a backward equalizer (BE) 34. A center tap c.sub.O of the overall DFE 30 is positioned at the final tap of the forward equalizer as shown. The forward equalizer 30 includes N-1 delay circuits 36 coupled in series, N multipliers 38 and an adder 40. On the other hand, the backward equalizer 34 is provided with M delay circuits 42, M multipliers 44 and an adder 46. Further, the DFE 30 includes a decision circuit 48, two subtractors 50, 52, and a tap control signal generator 54.
The DFE 30 is supplied with an incoming QAM signal (for example) and operates to minimize intersymbol interference (ISI) due to a precursor of an impulse response at the forward equalizer 32, while minimizing ISI caused by a postcursor at the backward equalizer 34. The output of the forward equalizer 32 is subtracted from the output of the backward equalizer 34 at the subtractor 50. The decision signal a.sub.n outputted from the decision circuit 48 and then fed back to the backward equalizer 34, is free of intersymbol interference and noises. Therefore, the equalization capability of the backward equalizer 34 using the decision feedback technique, is higher than that of the forward equalizer 32. This means that the backward equalizer 34 is capable of completely removing ISI caused by a postcursor of impulse response (viz., minimum phase shift fading). It goes without saying that the DFE 30 is superior to the case where only the forward equalizer 32 is provided which has the same function as the linear equalizer 10 shown in FIG. 1.
On the other hand, intersymbol interference due to a precursor (non-minimum phase shift fading) is equalized at the forward equalizer 32 whose function equals that of the linear equalizer 10 shown in FIG. 1. Consequently, in connection with the ISI due to non-minimum phase shift fading, the DFE 30 implements merely equalization which is identical to that of the forward equalizer 32. This is the reason why a easily installed linear equalizer 10 (or 32) is chiefly employed rather than a complex DFE in terrestrial digital microwave communications systems wherein severe distortion due to non-minimum phase fading occurs frequently.
A known approach to effectively removing intersymbol interference caused by non-minimum phase shift fading, is to provide a matched filter (MF) which is followed by a decision feedback equalizer (DEF) as shown in FIG. 2. The MF/DEF reception technique was proposed by Kojiro Watanabe in a paper in Japanese (CS78-203) entitled "Adaptive matched filter and its significance to anti-multipath fading", Feb. 22, 1979, presented at the Electronic Communications Association. The paper discloses adaptive signal reception by maximizing a signal-to-noise (SN) ratio together with equalization of distortion through the use of the DFE, thus improving the equalization capability of the DFE of intersymbol interference due to non-minimum phase shift fading. The paper further discusses a number of advantages derived from the usage of the matched filter.
FIG. 3 is a block diagram showing one example of the above-mentioned MF/DEF arrangement. The decision feedback equalizer (DFE) 30, which is identical to the arrangement illustrated in FIG. 2, is preceded by a matched filter 60 which includes a plurality of delay circuits 62 each having a symbol interval T/2, a plurality of multiplexers 64, and a tap control signal generator 68. As is well known, a matched filter maximizes the output ratio of peak signal power to mean noise power.
Response of a matched filter will briefly be described with reference to FIG. 4. A waveform (A) shown in FIG. 4 represents the impulse response before being processed by the matched filter (MF) 60, while a waveform (B) denotes impulse response of the MF 60. Since the matched filter 60 makes symmetrical the impulse response about a reference response point, a strong precursor in the range of t&lt;0 is partly converged to the reference response and the precursor component energy is dispersed over a range of 0&lt;t. Therefore, part of the precursor distortion due to the non-minimum phase fade is transferred to the postcursor distortion. This means that the forward equalizer 32 is able to lessen the burden thereon for equalizing the precursor distortion and the increased postcursor distortion is equalized at the backward equalizer 34. Accordingly, the overall equalization capability of the DFE 30 can be improved in connection with the precursor distortion equalization. It is understood that the MF/DFE exhibits an effective equalization capability against the precursor distortion relative to the arrangement consisting of the DEF only.
However, the above-mentioned MF/DFE is directed to maximization of a limited SN ratio using diversity techniques in an over-the-horizon digital communications system, and hence is unsuitable for a communications system wherein waveform distortion should effectively be equalized but a SN ratio is relatively high.
As mentioned previously, the MF/DFE exhibits excellent precursor distortion equalization performance as compared with the case where the DFE only is provided. However, the MF/DFE is inferior to the DFE in connection with the equalization of the postcursor distortion. This problem, inherent in the MF/DFE, is caused by new waveform distortion introduced by the provision of the MF 60 and is noticeable in the case of multi-level QAM system. This problem is enhanced with increase in the number of levels of QAM. More specifically, the impulse response undergoing the matched filtering converges at the reference position at t=0, but there exists the impulse response which disperse over a relatively wide range although the levels are low. In order to equalize such widely dispersed impulse responses, the number of taps of the MF should be increased to a considerable extent. However, this solution is very difficult or impossible to practically implement.